Graphviz is an open-source graph visualization software that allows users to create diagrams and visual representations of data structures. It uses a simple text-based language called DOT to describe the nodes and edges of a graph, making it easy to generate complex visualizations with minimal effort. The software is widely used in various fields, including computer science, bioinformatics, and project management, to illustrate relationships and hierarchies. Graphviz can produce output in multiple formats, such as PNG, PDF, and SVG, making it versatile for different applications and presentations.